(1) Field of the Invention
The invention relates to the formation of a protective outer layer on magnesium based alloys containing aluminum in a process utilizing a eutectic mixture of a molten salt held at an elevated temperature.
(2) Description of the Prior Art
The formation of a protective outer layer by electrolytic or electroless means on magnesium based alloys containing aluminum utilizing a bath of a fused salt has attracted little attention during the past two decades. On the other hand, the anodizing of aluminum in molten salts, mainly nitrates, has attracted some attention. Some of this work has been reviewed in Molten Salt Technology edited by D. G. Lovering, Plenum, N.Y. (1982) and Comprehensive Treatise of Electrochemistry Vol. 7, edited by J. O'M. Bockris, B. Conway and E. Yeager, Plenum, N.Y. (1983). With respect to the anodizing of aluminum, Lovering and Clark, Abstract No. 792, Extended Abstracts of the Electrochemical Society 83-1, Pennington, N.J. describe the anodizing of aluminum in a molten salt eutectic mixture containing 40.4 mole percent ammonium nitrate with the balance urea.
The electrolytic production of protective layers on magnesium and magnesium based alloys utilizing low melting mixtures of polyfluoride such as those melting below about 125.degree. centigrade, is disclosed in British Pat. No. 463,024. The heat treatment of magnesium alloys is disclosed in U.S. Pat. No. 2,036,563 utilizing a molten anhydrous compound consisting of sodium bichromate and potassium bichromate. There is also disclosed the prior art use of molten baths of potassium nitrate to anneal magnesium alloys.
There is no suggestion in these prior art references for the use of a low melting eutectic mixture of a nitrate or nitrate salt, an organic component, and water, specifically with a eutectic mixture of water, urea, and ammonium nitrate useful for the formation of a protective outer layer on a magnesium based alloy either by electrolytic means or electroless means.